If, in heterodyne detection, the light frequency is to be measured to a high frequency, the light to be measured is caused to interfere with other light and an electrical signal of the optical beat frequency generated is detected. The bandwidth of laser light that may be measured in this heterodyne detection is limited to the band of the light receiving element used in the detection system, and is generally on the order of tens of GHz.
On the other hand, the bandwidth of light that may be measured needs to be increased further in order to measure the frequency of absorption lines, distributed over a wide range, or in order to control laser light for frequency division multiplex communication, in keeping up with the development in the domain of optoelectronics in recent years.
With a view to responding to the demand for enlarging the measurable bandwidth of light, a broadband heterodyne detection system, employing an optical frequency comb generator, was already devised. This optical frequency comb generator generates a number of comb-shaped sidebands, arranged at an equal interval on the frequency axis. The frequency stability of the sidebands is substantially equivalent to the frequency stability of the incident light. The generated sidebands and the light being measured are heterodyne-detected to construct a broadband heterodyne detection system extending over several THz.
FIG. 1 shows the topical structure of a conventional optical frequency comb generator 9.
This optical frequency comb generator 9 includes an optical resonator 90, made up of an optical phase modulator 91 and reflecting mirrors 92, 93 arranged facing each other with the optical phase modulator 91 in-between.
The optical resonator 90 causes light resonation of light Lin, incident via reflecting mirror 92 with a low transmittance, in a space between the reflecting mirrors 92, 93, while radiating a fraction Lout of the incident light via reflecting mirror 93. The optical phase modulator 91 is formed by an electro-optical crystal for optical phase modulation, which is changed in refractive index on application of an electrical field thereto. The light traversing this optical resonator 90 is phase-modulated responsive to an electrical signal of the modulation frequency fm, supplied to an electrode 96.
By introducing an electrical signal, synchronized with the time of a round trip of light through the optical resonator 90, from the electrode 96 to the optical phase modulator 91 for driving, it is possible with this optical frequency comb generator 9 to apply phase modulation deeper tens of times than in case of light traveling only once through the optical phase modulator 91. Thus, the optical frequency comb generator 9 is able to generate hundreds of higher order sidebands. The frequency interval fm between the neighboring sidebands is equivalent the modulating frequency fm of the input electrical signals.
Meanwhile, in determining the frequency of the light under measurement based on the large number of the optical frequency combs generated, the optical frequency comb generator 9 modulates the incident light with the frequency ν1, with the frequency fm, by the optical phase modulator 91, to generate optical frequency combs composed of the sidebands with the frequency interval fm. These optical frequency combs are superposed on the light under measurement, with the frequency ν2, and the beat frequency Δν with respect to the Nth sideband generated as the optical frequency comb is measured to determine |ν1−ν2|. Ultimately, the frequency ν2 of the light under measurement is measured.
The light intensity distribution of the so generated sidebands is flattened out to render the sensitivity of the optical frequency combs constant for the entire frequency range, such that it becomes possible to measure the frequency of the light under measurement accurately such as to relieve the designing load in the downstream side circuitry used for detecting the generated sidebands.
However, in the conventional optical frequency comb generator 9, the light intensity of the sidebands is decreased with increase in the absolute value of Δν, in other words, with increase in the frequency deviation from the frequency of the incident light. In particular, the light intensity of the sidebands is exponentially decreased for a band which appreciably differs from the frequency of the incident light. The result is that the light intensity distribution of the sidebands is not uniform and susceptible to variations.
On the other hand, the optical frequency comb generator 9 has to use a reflecting mirror of high reflectance in order to suppress loss of light to be resonated. However, the reflecting mirror of high reflectance also reflects the light supplied from an external light source, thus increasing the light loss at the time of light incidence.
Thus, for accurately measuring the light under measurement, an optical frequency comb generator capable of suppressing the light loss to a minimum, as it is attempted to flatten out the light intensity distribution in the generated sidebands, needs to be realized.